The present invention relates to an improved dot printing element or apparatus which may be used either singly or in combination with other dot printing elements to form a dot matrix.
A prior art dot printing element to which the present invention constitutes a novel improvement comprises a housing in which is provided an electromagnetic coil. A ferromagnetic core is fixed to an end of a wire and is magnetically attractable by the core to move the free end of the wire into impact with paper through a ribbon to print a dot. The printing element may be steppingly moved to print a line of a dot matrix or may be combined with similar elements to print an entire character in, for example, a 5.times.7 dot matrix configuration.
A leaf spring fixed to the core is urged into engagement with a shoulder of a housing in which the coil and core are provided and returns the core and wire to a standby position after printing engagement with the paper. More specifically, the core is urged by the leaf spring into abutment with a stopper. The stopper is adjusted so that a preload is normally provided to the spring. A spring seat is provided between the shoulder of the housing and the leaf spring.
Over a period of use, the constant impact between the core and stopper causes wear of both components such that the preload of the leaf spring constantly decreases. Such impact causes cold working and shortening of the core and, to a lesser extent, the stopper. After a period of time the wear becomes so great that the preload of the leaf spring is reduced to zero and the spring seat, leaf spring and core are in a free state between the soulder of the housing and the stopper. Vibration of the apparatus during operation under these conditions causes the spring seat to axially move back and forth, thereby creating wear and the production of particles due to frictional abrasion. These particles intercede into the spaces between the stopper, core, leaf spring, spring seat and shoulder of the housing and interfere with the movement of the core and wire. Although these problems may be reduced by increasing the initial preload on the leaf spring, such is only a temporary solution. The amount of preload increase is furthermore limited since excessive preload will prevent the wire from moving through a sufficient distance and with sufficient velocity to print a dot on the paper.